The global positioning system (GPS) is a satellite based radio-navigation system built and operated by the United States Department of Defense. The Russian government operated ‘GLONASS’ and European Union proposed ‘Galileo’ are two other important satellite based navigational systems.
GPS permits a user of the system to determine his or her position on the surface of the earth. The system consists of twenty-four satellites circling the earth at an altitude of about 11,000 miles with a period of about 12 hours. It is possible to have more than twenty-four satellites due to the presence of some spare satellites in the GPS constellation. These satellites are placed in six different orbits such that at any time a minimum of six and a maximum of more than eleven satellites are visible to any user on the surface of the earth except in the polar region. Each satellite transmits an accurate time and position signal referenced to an atomic clock. A typical GPS receiver locks onto this signal and extracts the data contained in it and with signals from a sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
Sometimes GPS receivers are required to operate under very weak signal conditions as in foliage or indoors. In the present day practice, the receiver may get “assistance” in the form of additional acquisition aiding messages from a server or base station, or Internet based. But providing this type of assistance requires additional infrastructure and may not be available in all places. Also, the receiver requires additional hardware to receive the aiding messages. Therefore there is a need to develop GPS receivers that operate in “standalone” mode under weak or indoor signal conditions. Further, there is a need as in the case of E911 (Enhanced 911), for fast acquisition of the GPS signals. In addition to the above, the power saving in the receiver is also an important requirement.
Most of the standalone high sensitivity GPS receivers are based on a long non-coherent integration involving squaring loss and thus reducing the possible gain while taking a long time to acquire the satellite signal under weak signal conditions. U.S. Pat. No. 6,725,157 discloses a procedure wherein a GPS receiver first acquires the satellite signals and starts computing the position outdoor and then if possible maintains this lock when moved indoor. U.S. Pat. No. 6,757,610 discloses a method of storing some tracking/reacquisition aiding parameters such as clock Doppler and receiver velocity. However, these parameters help only in reducing the search frequency band. U.S. Pat. No. 6,683,564 discloses a technique of pattern matching using the known content of the navigation data word HOW, but this method involves getting the HOW from a local server. U.S. Pat. No. 5,768,319 discusses a method of multiple frame overlay to improve sensitivity. This overlay is done at the final stage and not with the I and Q components. U.S. Pat. No. 6,611,756 predicts the navigation data for long time integration. The predicted data includes mainly the time of week, week number, etc. The predicted data is used to wipe off the data modulation. U.S. Pat. No. 6,295,023 discloses a timing assistance scheme to improve sensitivity. The assistance may be from a network or from many satellites, but not by at least one satellite as will be shown in the invention and the implementation method is different from the scheme that will be presented in the invention. U.S. Pat. No. 6,424,890 claims a method of interpolation for satellite orbit determination at various time stamps. But there are no polynomial based ephemeris extrapolation methods disclosed. U.S. Pat. Nos. 5,731,787 and 5,587,716 disclose using polynomials to predict data only during DGPS blank-out period. U.S. Pat. No. 5,430,657 discloses a method of predicting the position of satellites using a plurality of receivers. This does not involve predicting the ephemeris but only used for testing whether ephemeris are corrupted or not. Further, published U.S. Patent Application 2005/0035904 discloses an ephemeris prediction based on tables. Finally, U.S. Pat. No. 4,601,005 discloses single satellite tracking using an FFT technique.
Thus, the prior art shows that there are no techniques available for the tracking and reacquisition of GPS signals in standalone mode especially in indoor environments and a method is required to reduce the cost and network interfacing problems. In addition, present day GPS receiver design techniques do not address the high dynamic environment under weak signal conditions in indoor environments.
Therefore, there is a need for a standalone GPS receiver capable of tracking and reacquiring GPS signals under weak signal conditions especially in indoor environments. Such a standalone GPS would also alleviate the cost and network interfacing problems by not requiring assistance from an external server or a network. There is also a need for a standalone GPS receiver having an efficient power saving mode while being capable of downloading the ephemeris and almanac whenever required. Further, there is a need for a standalone GPS receiver capable of operating in a high dynamic environment under weak signal conditions.
Fast reacquisition of GPS signals is needed in many applications. Thus, there is a need for updating the ephemeris data in a receiver, which helps in accurate estimation of the distance from receiver to satellite for Doppler estimation, predicting data for pattern matching, ect. There is also a need for long coherent integration instead of time consuming non-coherent integration. Continuous tracking of satellite when visible satellites list changes is also important. When no direct satellite signals are available, there is a need for position computation means based on less accurate multi-path signals. Further, there is a need to make use of outdated ephemeris rather than almanacs for better accuracy.